CN110334424B - Aerodynamic performance prediction method reflecting air permeability of parachute structure - Google Patents

Aerodynamic performance prediction method reflecting air permeability of parachute structure Download PDF

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CN110334424B
CN110334424B CN201910559401.3A CN201910559401A CN110334424B CN 110334424 B CN110334424 B CN 110334424B CN 201910559401 A CN201910559401 A CN 201910559401A CN 110334424 B CN110334424 B CN 110334424B
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predicted
fabric
air permeability
flow field
parachute
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CN110334424A (en
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余莉
聂舜臣
赵晓舜
庞昊裴
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Nanjing University of Aeronautics and Astronautics
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Nanjing University of Aeronautics and Astronautics
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    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F30/00—Computer-aided design [CAD]
    • G06F30/10—Geometric CAD
    • G06F30/17—Mechanical parametric or variational design
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F2111/00—Details relating to CAD techniques
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    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F2113/00—Details relating to the application field
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    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T90/00—Enabling technologies or technologies with a potential or indirect contribution to GHG emissions mitigation

Abstract

The invention relates to a pneumatic performance prediction method reflecting the air permeability of a parachute structure, which only needs to build a parachute structural model in blocks on the basis of the pneumatic appearance of a general parachute, wherein the air permeability of a fabric and the air permeability of the structure of a completely open hole part are expressed by relative air permeability, and the relative air permeability is expressed by an air permeability coefficient based on a porous medium theory. On the basis, a ventilation domain flow field model is established, and the ventilation domain thickness is determined by jet theory. And then, the aerodynamic performance of the parachute is predicted by numerical calculation of the ventilation domain flow field model and the general domain flow field model. The method can conveniently adjust the fabric air permeability of each part of the parachute canopy, and more importantly, the method does not need to remodel various slotted parachutes, so that the modeling efficiency is greatly improved, and the method has extremely important significance for the prediction and the optimization design of the aerodynamic performance of the parachute.

Description

Aerodynamic performance prediction method reflecting air permeability of parachute structure
Technical Field
The invention relates to a pneumatic performance prediction method reflecting the air permeability of a parachute structure, and belongs to the technical field of local pneumatic performance test of parachutes.
Background
In order to improve stability, reduce the dynamic load of parachute opening and improve the performance of maneuvering and gliding, a certain structure of ventilation holes are normally arranged on the parachute. The slotting part and slotting mode of the parachute are various: the machine tool is provided with a warp slit, a weft slit, a symmetrical slit and an asymmetrical slit. The structure ventilation holes have larger influence on the aerodynamic performance (such as resistance coefficient, stability and parachute opening performance) of the parachute (Ganzin, rong Wei. Sensitivity analysis of variable parameters of the annular parachute to design deviation [ J ]. Space return and remote sensing, 2014,35 (02): 14-24). The current theoretical prediction of parachute aerodynamic performance is mainly realized based on solving an N-S equation based on computational fluid dynamics (McQuilling, M.W.and Potvin J. "CFD Simulations of a Double-Annulus Parachute System for Cargo Airdrops, using Inflated Shapes Informed by Photogrammetry",23rd AIAA Aerodynamic Decelerator Systems Technology Conference in Daytona Beach,the American Institute ofAeronautics andAstronautics,Virginia,AIAA2015-2169, pp.1-10), and the method does not consider the air permeability of the fabric and has a great error.
In order to fully simulate the air permeability of the canopy fabric material, a learner uses a porous medium jump model (Chen Xiaodong, zhou Yuncheng, wang Depeng) to conduct numerical simulation on the parachute with fabric air permeability by using a macroscopic fluid analysis [ J ]. Wool spinning technology, 2016,44 (05): 5-8), a fabric air permeability field flow field model (Zheng G., richard D. Charles, and Xiaolin L. (2017), "Numerical Modeling ofFlow Through Porous Fabric Surface in Parachute Simulation", AIAA Journal, vol.55, no.2, pp.686-690), respectively, to improve the accuracy of the pneumatic performance analysis, but no determination method of the air permeability field is given. It is particularly worth noting that the above method is merely for more accurately simulating the fabric breathability of the canopy, and for the canopy structure to open, it is necessary to build a canopy model in a true open state. In the design process, if the position or the area of the opening of the umbrella canopy needs to be adjusted, the modeling is needed to be performed again for each change, so that the modeling workload is greatly increased, and the efficiency of design analysis is reduced.
Disclosure of Invention
The technical problem to be solved by the invention is to provide the pneumatic performance prediction method for reflecting the air permeability of the parachute structure, which aims at the perforated parachute canopy at different positions, does not need to carry out structural modeling again, and can accurately consider the air permeability of the structure and the air permeability of materials at different positions, thereby improving the efficiency of the optimized design of the parachute.
The invention adopts the following technical scheme for solving the technical problems: the invention designs a pneumatic performance prediction method reflecting the air permeability of a parachute structure, which is used for dividing structural characteristics of a gore into blocks as objects to be predicted, establishing a calculation model for carrying out pneumatic performance prediction, and comprises the following steps of:
step A. According to the relative air permeabilityB, obtaining a theoretical value a of a viscosity coefficient and a theoretical value B of an inertia coefficient of the air permeability of the fabric of the object to be predicted, and then entering a step B;
step B, obtaining the theoretical actual proportion k of the fabric to be predicted according to the ratio of the theoretical thickness delta of the fabric to be predicted to the preset actual thickness e of the fabric to be predicted, and then entering the step C;
step C, according to the theoretical actual proportion k of the fabric of the object to be predicted, combining the values a and b to obtain an actual value a 'of the air permeability viscosity coefficient and an actual value b' of the inertia coefficient of the fabric of the object to be predicted, and then entering the step D;
step D, obtaining a normal momentum source item S along the surface of each area on the object to be predicted according to the following formula i Then enter step E;
S i ＝a′v i +b′|v|v i
wherein v is i Representing a normal velocity component of a region to be predicted on an object to be predicted; v represents the relative speed between the ring sail umbrella and the air flow;
e, respectively constructing a ventilation domain flow field model along the normal direction aiming at the region to be predicted on the object to be predicted, wherein the ventilation domain flow field model is as follows:
wherein ρ is the fluid density; t is time; div () is a divergence calculation function; grad is gradient calculation; μ is the fluid viscosity coefficient; p is flow field pressure; x is x i For the object to be predicted i-directional coordinates,representing partial derivative operations;
meanwhile, aiming at other areas of the object to be predicted, a general domain flow field model is built according to a conventional N-S equation, and then the step F is carried out;
and F, carrying out flow field numerical calculation according to a general domain flow field model corresponding to the object to be predicted and an air permeability domain flow field model corresponding to each area surface normal direction on the object to be predicted, so as to realize the prediction of the aerodynamic performance of the object to be predicted.
As a preferred technical solution of the present invention, the step a includes the following steps:
step A1. Constructing the air permeability coefficient of the fabric of the object to be predicted and the relative air permeability of the fabric of the object to be predictedThe mathematical model is as follows, and then the step A2 is carried out;
wherein v represents the relative speed between the ring sail umbrella and the air flow, and L represents the fabric thickness of the object to be predicted;
a2, obtaining the relationship between the theoretical value a of the air permeability viscosity coefficient and the theoretical value b of the inertia coefficient of the fabric of the object to be predicted according to the porous medium permeation theory as follows, and then entering a step A3;
wherein f 1 ()、f 2 () Respectively representing correction functions of the theoretical value of the air permeability viscosity coefficient and the theoretical value of the inertia coefficient of the fabric, which are influenced by the porosity of the fabric, wherein mu represents hydrodynamic viscosity at normal temperature;
and A3, substituting the formula (5) into the formula (3) to obtain a theoretical value b of the air permeability coefficient of the fabric to be predicted, and then obtaining a theoretical value a of the air permeability coefficient of the fabric to be predicted according to the formula (5).
As a preferred technical solution of the present invention, the step B includes the following steps:
obtaining the length delta of the converging areas of the multiple jet streams corresponding to the object to be predicted according to the following formula 0 Then go to step B2;
δ 0 ＝5.06D(D 0 /D) 0.27
wherein D is 0 Representing the distance between the centers of two adjacent yarns of the fabric to be predicted, and D represents the diameter of the pores of the fabric to be predicted;
b2, obtaining the theoretical thickness delta of the fabric of the object to be predicted according to the following formula, and then entering a step B3;
δ＝L+δ 0
and step B3, obtaining the theoretical actual proportion k of the fabric to be predicted according to the ratio of the theoretical thickness delta of the fabric to be predicted to the preset actual thickness e of the fabric to be predicted.
As a preferred technical scheme of the invention: in the step C, according to the theoretical actual proportion k of the object fabric to be predicted, the following formula is adopted:
and obtaining the actual value a 'of the air permeability viscosity coefficient and the actual value b' of the inertia coefficient of the fabric of the object to be predicted.
As a preferred technical scheme of the invention: in the step E, according to the general domain flow field model corresponding to the object to be predicted and the air permeability domain flow field model corresponding to each area surface normal direction on the object to be predicted, carrying out flow field numerical calculation on the object to be predicted, and when the fabric relative air permeability of the object to be predicted is calculatedWhen the porosity epsilon of the fabric of the object to be predicted is equal to the porosity epsilon of the fabric of the object to be predicted, the pneumatic performance of the weaving area of the object to be predicted is predicted; when the relative air permeability of the object fabric to be predicted is +.>And when the air pressure is equal to 1, the air pressure performance of the opening area of the object to be predicted is predicted.
Compared with the prior art, the pneumatic performance prediction method for reflecting the air permeability of the parachute structure has the following technical effects:
according to the pneumatic performance prediction method reflecting the air permeability of the parachute structure, structural modeling is not required to be carried out again aiming at the perforated parachute coatings at different positions, and the influence of the local perforated structure on the pneumatic performance of the parachute can be accurately reflected; therefore, the relative air permeability is used for representing the air permeability of the materials at different parts of the parachute, errors caused by measuring the pressure drop of the fabric are eliminated, the air permeability prediction process of the fabric of the parachute is simplified, and therefore the efficiency of the optimal design of the parachute is improved.
Drawings
FIG. 1 is a schematic diagram of a method for predicting aerodynamic performance reflecting the air permeability of a parachute structure designed in accordance with the present invention.
FIG. 2 is a schematic plan view of a circular sail umbrella canopy;
FIG. 3 is a schematic diagram of a model A, C;
FIG. 4 is a schematic diagram of model B;
FIG. 5 is a flow field domain and porous media domain mesh of model A, C;
FIG. 6 is a grid plot of the flow field domain and porous media domain of model B;
fig. 7a, 7b, and 7c are, in order, flow field velocity vector diagrams and partial enlarged diagrams of the model A, B, C;
FIG. 8 is a graph of the pressure profile along the meridian of the canopy of model A, B;
FIG. 9 is a graph of the pressure profile along the meridian of a model A, B ring patch;
FIG. 10 is a graph of the pressure profile along the meridian of the canopy of model A, C;
fig. 11 is a pressure profile along the meridian of the model A, C patch.
Detailed Description
The following describes the embodiments of the present invention in further detail with reference to the drawings.
The invention designs a pneumatic performance prediction method reflecting the air permeability of a parachute structure, which is used for dividing structural characteristics of a gore into blocks to be used as an object to be predicted, establishing a calculation model for carrying out pneumatic performance prediction, and particularly executing the following steps in practical application as shown in figure 1.
Step A. According to the relative air permeabilityAnd (C) obtaining a theoretical value a of the viscosity coefficient and a theoretical value B of the inertia coefficient of the air permeability of the fabric of the object to be predicted, and then entering the step B.
In practical application, the following procedure is specifically performed in the above step a.
First, defining the fabric phase of the object to be predictedTo air permeabilityThe following are provided:
in the formula, v q For the fabric ventilation of the object to be predicted, v represents the relative speed between the ring sail and the air flow.
Then, the air permeability coefficient of the fabric to be predicted and the relative air permeability of the fabric to be predicted are constructedThe mathematical model between them, i.e. step A1 as follows.
Step A1. Introducing a Forchheimer formula in a porous medium seepage theory:
wherein the pressure difference isCan be expressed as:
relative air permeability of object fabric to be predictedSubstituting the formula (1) to obtain the formula (3), and then proceeding to step A2.
Where v denotes the relative speed between the ring sail umbrella and the air flow, L denotes the fabric thickness of the object to be predicted, ρ denotes the fluid density.
According to the porous medium permeation theory, the theoretical value a of the air permeability viscosity coefficient and the theoretical value b of the inertia coefficient of the fabric to be predicted are obtained to meet the following formula:
wherein f 1 ()、f 2 () The modified functions of the theoretical value of the air permeability viscosity coefficient and the theoretical value of the inertia coefficient of the fabric, which are affected by the porosity of the fabric, are respectively represented, and can be described by referring to a nonlinear motion equation of a porous medium, mu represents hydrodynamic viscosity at normal temperature, and D represents the pore diameter of the fabric to be predicted.
Based on the above formula (3), the following step A2 is performed.
And step A2, obtaining the relationship between the theoretical value a of the air permeability viscosity coefficient and the theoretical value b of the inertia coefficient of the fabric of the object to be predicted according to the porous medium permeation theory, and then entering the step A3.
And step A3, substituting the formula (5) into the formula (3) to obtain the following formula (6):
obtaining a theoretical value b of the air permeability inertia coefficient of the fabric to be predicted, wherein f 3 () Is according to ρ, L, v, And b is a function of the dependent variable, and then according to a formula (5), the theoretical value a of the air permeability and viscosity coefficient of the fabric to be predicted is obtained.
And B, obtaining the theoretical actual proportion k of the fabric of the object to be predicted according to the ratio of the theoretical thickness delta of the fabric of the object to be predicted to the preset actual thickness e of the fabric of the object to be predicted, and then entering the step C.
In practical implementation, the step B specifically includes the following steps.
Obtaining the length delta of the converging areas of the multiple jet streams corresponding to the object to be predicted according to the following formula 0 Then step B2 is entered.
δ 0 ＝5.06D(D 0 /D) 0.27 (7)
Wherein D is 0 And D represents the diameter of the pores of the fabric to be predicted.
And B2, obtaining the theoretical thickness delta of the fabric of the object to be predicted according to the following formula, and then entering a step B3.
δ＝L+δ 0 (8)
And B3, according to the ratio of the theoretical thickness delta of the fabric to be predicted to the preset actual thickness e of the fabric to be predicted, the method comprises the following steps:
and obtaining the theoretical actual proportion k of the fabric of the object to be predicted.
Due to the fabric thickness of the order of magnitude of 10 -4 m, while the nominal diameter of the canopy is in the order of meters or even hundreds of meters (such as a main parachute of a Shenzhou airship), if the theoretical thickness value is adopted for calculation, the number of grids is too large, so that the waste of calculation resources is avoided, and the theoretical actual proportion k of the fabric of the object to be predicted is introduced.
And C, combining the values of a and b according to the theoretical actual proportion k of the fabric of the object to be predicted, and adopting the following formula:
and obtaining an actual value a 'of the air permeability viscosity coefficient and an actual value b' of the inertia coefficient of the fabric of the object to be predicted, and then entering a step D.
Step D, obtaining a normal momentum source item S along the surface of each area on the object to be predicted according to the following formula i Then enter step E;
S i ＝a′v i +b′|v|v i (11)
wherein v is i Representing a normal velocity component along a prediction area on an object to be predicted; v denotes the relative speed between the ring sail umbrella and the air flow.
E, respectively constructing a ventilation domain flow field model aiming at the normal direction of the area to be predicted on the object to be predicted, wherein the ventilation domain flow field model is as follows:
in the process, S is removed i The other parts are conventional flow field momentum control equations, where ρ is the fluid density; t is time; div () is a divergence calculation function; grad is gradient calculation; μ is the fluid viscosity coefficient; p is flow field pressure; x is x i For the object to be predicted i-directional coordinates,representing the partial derivative operation.
Meanwhile, aiming at other areas of the object to be predicted, a general domain flow field model is built according to a conventional N-S equation, and then the step F is carried out.
And F, carrying out flow field numerical calculation according to a general domain flow field model corresponding to the object to be predicted and an air permeability domain flow field model corresponding to each area surface normal direction on the object to be predicted, so as to realize the prediction of the aerodynamic performance of the object to be predicted.
In a specific practical application, in the step F, when the relative air permeability of the fabric of the object to be predicted isEqual to the porosity of the fabric of the object to be predictedAnd epsilon, realizing the prediction of the pneumatic performance of the weaving area of the object to be predicted; when the relative air permeability of the object fabric to be predicted is +.>And when the air pressure is equal to 1, the air pressure performance of the opening area of the object to be predicted is predicted.
The above designed method for predicting aerodynamic performance reflecting the air permeability of the parachute structure is applied to practice, the plane structure of the annular sail parachute canopy is shown in fig. 2, the canopy comprises 3 annular sheets and 6 sail sheets, and specific structural dimensions are shown in the following table 1.
TABLE 1
Firstly, an umbrella canopy structure model is built according to structural characteristics of a ring sail umbrella and pneumatic appearance blocks when the ring sail umbrella is full, in the embodiment, the umbrella canopy is divided into 4 blocks for modeling, namely a ring piece 1, a ring piece 2, a ring piece 3 and a sail piece, as shown in fig. 3, a calculation model is built specifically aiming at the ring piece 2 as an object to be predicted, and pneumatic performance prediction is carried out.
And then respectively constructing a basic model and two groups of comparison models to verify the influence effect of the air permeability parameters on the aerodynamic performance.
The basic model A is shown in FIG. 3, in which the relative air permeability of the fabric of the middle ring 2 is set
The comparative model B is shown in fig. 4, in which the ring segment 2 is partially removed based on fig. 3, namely, the structure hole opening mode.
Comparative model C As shown in FIG. 3, the ring segment 2 is made of conventional canopy material, and has relative air permeability
The air permeability of the three models of other materials is completely consistent, and the relative air permeability of the materials of each ring piece and the sail pieceBoth 3.8%.
According to relative air permeabilityAnd (C) obtaining a theoretical value a of the viscosity coefficient and a theoretical value B of the inertia coefficient of the air permeability of the fabric of the object to be predicted, and then entering the step B.
Wherein, the formula of Forchheimer introduced into the porous medium seepage theory:
wherein the pressure difference isCan be expressed as:
relative air permeability of object fabric to be predictedSubstituting the formula (1) to obtain the formula (3).
Wherein a and b are respectively the theoretical value of the viscosity coefficient and the theoretical value of the inertia coefficient of the air permeability of the fabric to be predicted, L represents the thickness of the fabric to be predicted, and a and b satisfy the following formulas:
wherein f 1 ()、f 2 () Respectively represent the ventilation of fabricsThe modification function of the theoretical value of the coefficient of performance viscosity and the theoretical value of the coefficient of inertia, which are affected by the porosity of the fabric, can be described with reference to the nonlinear equation of motion of the porous medium, μ representing the hydrodynamic viscosity at normal temperature, D representing the pore diameter of the fabric of the object to be predicted. The relationship with reference to the porous media theory Ergun formula, and fabric porosity, can be written as follows:
where ρ represents the fluid density. According to formula (3), D is eliminated, and according to the porous medium permeation theory, the relationship between the theoretical value a of the air permeability viscosity coefficient and the theoretical value b of the inertia coefficient of the fabric to be predicted is obtained as follows
Order theSubstituting formula (6) into formula (3), and writing the following formula:
for air ρ=1.225 kg/m 3 μ=1.76e-5 pa·s, v=15 m/s, l=0.3 mm can be solved:
according to the relative air permeability of 3.8% of the gores, the theoretical value of the viscosity coefficient a and the theoretical value of the inertia coefficient b (except for the ring piece 2) of the air permeability of the gore fabric are: a=5.47 e 4 kg/m 3 s,b＝1.32e 6 kg/m 4 . For the loop sheet 2, the model a here sets the relative air permeability of the fabric to 1, so a=b=0, for ease of numerical calculation,a, b can be taken to be infinitesimal, where a=1e is taken to be -6 kg/m 3 s,b＝1e -6 kg/m 4 The method comprises the steps of carrying out a first treatment on the surface of the Model B was fully open at this location (fig. 4), with no structural block canopy present; the fabric tack and inertia coefficients of model C are the same as the other blocks at this location, namely: a=5.47 e 4 kg/m 3 s,b＝1.32e 6 kg/m 4 。
And determining the thickness of the ventilation field, and establishing a ventilation field flow field model and a general field flow field model.
Considering that fluid flows through the canopy like a porous jet, the thickness of the jet affected area is determined by the jet intersection, which thickness can be expressed according to jet theory as:
δ 0 ＝5.06D(D 0 /D) 0.27 (9)
in the formula, referring to experimental data of fabrics, the embodiment enables D 0 For 200um, d is 38um, calculated as: delta 0 =0.3 mm. The theoretical thickness delta of the object fabric to be predicted is:
δ＝L+δ 0 (10)
substitution value is: δ=0.6 mm.
Thereby establishing a ventilation domain flow field model and a general domain flow field model according to the outline of the umbrella canopy, as shown in fig. 5-6. The ventilation domain flow field model is as follows:
wherein S is i The normal momentum source term for each area surface on the object to be predicted is determined by the following formula:
S i ＝a′v i +b′|v|v i (12)
in the formula, v i For the i-direction velocity component, the fabric thickness is of the order of 10 -4 m, while the nominal diameter of the canopy is in the order of meters or even hundreds of meters (such as the main umbrella of a Shenzhou airship), if the theoretical thickness value is adopted for calculation, the number of grids is too large, so as to avoid the waste of calculation resources,let the similarity ratio k=1/50, a 'and b' represent the actual values of the air permeability and viscosity coefficients and the actual values of the inertia coefficients of the fabric of the object to be predicted, respectively, and can be determined by the following formula:
in the formula, e is the preset actual thickness of the fabric of the object to be predicted, namely the normal thickness of the porous medium domain grid, namely the calculated domain actual thickness, is 30mm. The calculation can be as follows: a' =2e -8 kg/m 3 s,b′＝2e -8 kg/m 4 (model a), a' =1.1e 3 kg/m 3 s，b′＝2.64e 4 kg/m 4 (model C).
And carrying out parachute bypass flow field calculation according to the computational fluid mechanics basic theory and method. Flow field velocity vector diagrams of the three models were obtained, as shown in fig. 7a to 7c, and pressure distribution diagrams of the model A, B along the meridian of the canopy surface were shown in fig. 8 (the horizontal axis represents the ratio of the abscissa of the canopy surface along the meridian to the radius of the bottom edge of the canopy), and the canopy resistance coefficients Cd of the three models are shown in table 2 below.
TABLE 2
Model A Model B Model C Model C experimental values
Cd 0.6631 0.6608 0.7525 0.733
From this, it can be seen that the flow field velocity vector diagrams of model a and model B are almost identical, and as shown in fig. 7a and 7B, the air flow completely penetrates the canopy through the ring sheet 2, the pressure difference between the inside and outside of the canopy on the ring sheet 2 is almost 0, and the inside and outside pressure lines of the two almost coincide, and the drag coefficients differ by only 0.35% (table 2). The method for adopting the relative air permeability of the fabric can accurately reflect the flow field rule and the aerodynamic performance of the parachute opening part, does not need to model various slotted parachutes again, and greatly improves the modeling efficiency.
In the flow field velocity vector diagram of model C, only a small portion of the airflow passes through the annular sheet 2 and penetrates the canopy, and the pressure difference between the inside and outside of the canopy on the annular sheet 2 is greater than that of model a, so that the drag coefficient is greater than that of model a. Therefore, the invention can conveniently adjust the material parameters in blocks, thereby achieving ideal pneumatic performance.
The model C numerical calculation result is compared with the air drop test data, and the resistance coefficient error of the model C numerical calculation result and the air drop test data is only 2.7%, so that the model C numerical calculation result shows that the model C numerical calculation result can accurately reflect the pneumatic performance of the parachute fabric under the condition of ventilation, and has extremely important significance for the optimized design of the parachute.
According to the pneumatic performance prediction method reflecting the air permeability of the parachute structure, which is designed by the technical scheme, structural modeling is not required to be carried out again aiming at the perforated parachute coatings at different positions, so that the influence of the local perforated structure on the pneumatic performance of the parachute can be accurately reflected; therefore, the relative air permeability is used for representing the air permeability of the materials at different parts of the parachute, errors caused by measuring the pressure drop of the fabric are eliminated, the air permeability prediction process of the fabric of the parachute is simplified, and therefore the efficiency of the optimal design of the parachute is improved.
The embodiments of the present invention have been described in detail with reference to the drawings, but the present invention is not limited to the above embodiments, and various changes can be made within the knowledge of those skilled in the art without departing from the spirit of the present invention.

Claims (4)

1. The pneumatic performance prediction method reflecting the air permeability of the parachute structure is used for dividing structural characteristics of the gores into blocks to be predicted, establishing a calculation model for carrying out pneumatic performance prediction, and is characterized by comprising the following steps of:
step A. According to the relative air permeabilityB, obtaining a theoretical value a of a viscosity coefficient and a theoretical value B of an inertia coefficient of the air permeability of the fabric of the object to be predicted, and then entering a step B;
in the step A, the relative air permeability of the fabric to be predicted is defined firstThe following are provided:
in the formula, v q The ventilation quantity of the fabric of the object to be predicted is v, and the relative speed between the ring sail umbrella and the air flow is represented;
then constructing the air permeability coefficient of the fabric to be predicted and the relative air permeability of the fabric to be predictedThe mathematical model between the two steps is executed as follows from step A1 to step A3;
step A1. Introducing a Forchheimer formula in a porous medium seepage theory:
wherein the pressure difference isCan be expressed as:
relative air permeability of object fabric to be predictedSubstituting the air permeability coefficient of the fabric to be predicted and the relative air permeability of the fabric to be predicted into the formula (1) to obtain the formula (3)>The mathematical model between is as follows:
wherein v represents the relative speed between the ring sail umbrella and the air flow, L represents the fabric thickness of the object to be predicted, and ρ represents the fluid density;
according to the porous medium permeation theory, the theoretical value a of the air permeability viscosity coefficient and the theoretical value b of the inertia coefficient of the fabric to be predicted are obtained to meet the following formula:
wherein f 1 ()、f 2 () Respectively representing correction functions of the theoretical value of the air permeability viscosity coefficient and the theoretical value of the inertia coefficient of the fabric, which are influenced by the porosity of the fabric, describing the correction functions by referring to a nonlinear motion equation of a porous medium, wherein mu represents hydrodynamic viscosity at normal temperature, D represents the pore diameter of the fabric of an object to be predicted, and then entering a step A2;
a2, obtaining the relationship between the theoretical value a of the air permeability viscosity coefficient and the theoretical value b of the inertia coefficient of the fabric of the object to be predicted according to the porous medium permeation theory as follows, and then entering a step A3;
and step A3, substituting the formula (5) into the formula (3) to obtain the following formula (6):
obtaining a theoretical value b of the air permeability inertia coefficient of the fabric to be predicted, wherein f 3 () Is according to ρ, L, v, B is a function of the dependent variable, and then according to a formula (5), the theoretical value a of the air permeability viscosity coefficient of the fabric to be predicted is obtained;
step B, obtaining the theoretical actual proportion k of the fabric to be predicted according to the ratio of the theoretical thickness delta of the fabric to be predicted to the preset actual thickness e of the fabric to be predicted, and then entering the step C;
step C, according to the theoretical actual proportion k of the fabric of the object to be predicted, combining the values a and b to obtain an actual value a 'of the air permeability viscosity coefficient and an actual value b' of the inertia coefficient of the fabric of the object to be predicted, and then entering the step D;
step D, obtaining a normal momentum source item S along the surface of each area on the object to be predicted according to the following formula i Then enter step E;
S i ＝a′v i +b′|v|v i
wherein v is i Representing a normal velocity component along a prediction area on an object to be predicted; v represents the relative speed between the ring sail umbrella and the air flow;
e, respectively constructing a ventilation domain flow field model aiming at the normal direction of the area to be predicted on the object to be predicted, wherein the ventilation domain flow field model is as follows:
wherein ρ is the fluid density; t is time; div () is a divergence calculation function; grad is gradient calculation; μ is the fluid viscosity coefficient; p is flow field pressure; x is x i For the object to be predicted i-directional coordinates,representing partial derivative operations;
meanwhile, aiming at other areas of the object to be predicted, a general domain flow field model is built according to a conventional N-S equation, and then the step F is carried out; and F, carrying out flow field numerical calculation according to a general domain flow field model corresponding to the object to be predicted and an air permeability domain flow field model corresponding to each area surface normal direction on the object to be predicted, so as to realize the prediction of the aerodynamic performance of the object to be predicted.
2. The method for predicting aerodynamic performance reflecting air permeability of a parachute construction according to claim 1, wherein the step B comprises the steps of:
obtaining the length delta of the converging areas of the multiple jet streams corresponding to the object to be predicted according to the following formula 0 Then go to step B2;
δ 0 ＝5.06D(D 0 /D) 0.27
wherein D is 0 Representing the distance between the centers of two adjacent yarns of the fabric to be predicted, and D represents the diameter of the pores of the fabric to be predicted;
b2, obtaining the theoretical thickness delta of the fabric of the object to be predicted according to the following formula, and then entering a step B3;
δ＝L+δ 0
and step B3, obtaining the theoretical actual proportion k of the fabric to be predicted according to the ratio of the theoretical thickness delta of the fabric to be predicted to the preset actual thickness e of the fabric to be predicted.
3. The method for predicting aerodynamic performance reflecting air permeability of a parachute structure according to claim 1, wherein: in the step C, according to the theoretical actual proportion k of the object fabric to be predicted, the following formula is adopted:
and obtaining the actual value a 'of the air permeability viscosity coefficient and the actual value b' of the inertia coefficient of the fabric of the object to be predicted.
4. A method for predicting aerodynamic performance reflecting air permeability of a parachute construction according to any one of claims 1 to 3, characterized in that: in the step E, according to the general domain flow field model corresponding to the object to be predicted and the air permeability domain flow field model corresponding to each area surface normal direction on the object to be predicted, carrying out flow field numerical calculation on the object to be predicted, and when the fabric relative air permeability of the object to be predicted is calculatedWhen the porosity epsilon of the fabric of the object to be predicted is equal to the porosity epsilon of the fabric of the object to be predicted, the pneumatic performance of the weaving area of the object to be predicted is predicted; when the relative air permeability of the object fabric to be predicted is +.>And when the air pressure is equal to 1, the air pressure performance of the opening area of the object to be predicted is predicted.
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