RU2425344C1 - Device for determination of forces of added inertia and damping bodies by method of their free damping vibrations in liquid - Google Patents

Abstract

FIELD: machine building. ^ SUBSTANCE: device consists of model of vessel, of platform facilitating their free vibrations around stationary supports, instrumentation for measurement of angle of platform deviation from balanced position and record of its free vibrations in time upon deviation and of hydraulic tray. Models are rolled by means of the platform whereto they are attached from beneath. The platform is equipped with telescopic rods supporting on ends horizontal axis with bearings inserted into ends of telescopic rods of foundation structure resting on walls of the hydraulic tray. Level of hydraulic tray filling can be efficiently changed. Frequency of platform vibrations in air and water can be changed by changing length of platform and ballast suspension. Required results are obtained as difference between inertia moments and shock absorption of structure assembly in water with different frequencies and in air. ^ EFFECT: expanded functionality of device. ^ 4 dwg

Description

The proposed technical solution belongs to laboratory facilities designed to determine the forces of attached inertia and damping of bodies of various shapes with their free damped vibrations in a liquid. It is recommended for use in shipbuilding and hydraulic engineering related to the calculation of ship rolls and the impact of waves on stationary offshore structures.

There is a known method of vessel heeling under full-scale conditions and their models in laboratory conditions, as a result of which their transverse stability coefficients k _Θ , = gρVh are determined, where V is the displacement of the vessel (model) and h is the metacentric height. Then the ships or models are rocked and the process of their onboard damped oscillations is recorded (Handbook of ship theory "Statics of ships. Ship pitching" vol. 2. / Edited by Y.I. Voitkunsky. - L .: Sudostroenie, 1985, p. 417) - analogue 1. The disadvantage of these methods is that they allow you to determine the attached moments of inertia λ ₄₄ and damping λ ₄₄ only with the onboard vibrations of the vessel (models) around a longitudinal horizontal axis passing through its center of mass, and only at one frequency. However, it is known that they depend on the oscillation frequency (wave frequency). In addition, this method does not allow to determine the attached mass λ ₃₃ and the damping force µ ₃₃ with the horizontal action of the waves in the side. And they are also needed when calculating the effects of waves on offshore structures and ship pitching.

A known method for determining the moment of inertia of a ship model, in which it is suspended on a flexible bifilar cable of length l in air, is rotated through an angle φ and released (the above reference page 416). Under the action of the restoring moment, the model performs damped oscillations, the period of which m is recorded. The moment of inertia about the vertical axis passing through the center of mass G is determined by the formula I _yy = I _zz = Pa ² τ ² / (4π ² l), where P = gM is the model’s gravity and a is the distance between the OY axis and the bifilars. On bifilars, it is possible to swing the model in the transverse direction and determine the moment of its inertia relative to the level of fastening of the bifilars - analog 2. But this analog does not allow determining the forces and moments of the attached inertia and damping in the liquid, because in water, the gravity of the model P is balanced by the forces of its buoyancy P _A = gρV, while there are no regenerative moments, and therefore the proper pumping of the models is impossible. However, with a certain modernization of this technical solution, it can be applied also with body vibrations in water.

Closest to the proposed technical solution is a device for determining the position of the center of mass and moments of inertia of large objects [2]. It includes a platform - the carrier of these objects, which is suspended on 4 chains. This platform is preliminarily weighed and the methods of static calibration on the chains determine the position of its center of mass, and then by the fluctuations - its moment of inertia. The studied object is placed and fixed on the platform, a static calibration of the platform with the object is carried out, and the position of the center of mass of the object is determined by known methods. Next, the platform with the object is rejected and released. Its damped oscillations are recorded and the moment of inertia of the platform with the object is determined by the period of these oscillations. Knowing the intrinsic mass and moment of inertia of the platform, the known methods determine the moment of inertia of the prototype object under study. The disadvantage of the prototype is that tests are carried out in air and at the same frequency, and, in addition, this device is not adapted to determine the moments of inertia of bodies in water.

The purpose of the invention is the expansion of the functionality of known technical solutions. This goal is achieved by the fact that in known technical solutions, including a model of a vessel or other body, a platform for providing their own vibrations around fixed supports, equipment for measuring the angle of deviation of the platform from its equilibrium position and recording its free vibrations in time after deflection and hydrotracks, the platform made in the form of a rigid frame, for example, of metal squares, equipped with calibration weights, a model is attached to it at different angles from the bottom, 2 strong sliding rods made, for example, of n-shaped profiles inserted into each other, the lower ends of which are firmly, for example, welded, connected to the frame, and the upper ends are rigidly connected to the axis, with which the platform itself with calibration weights, together with model or with additionally accepted loads can fluctuate in bearings at the ends of this axis, inserted into special sockets (limiters) located in the foundation structure, each platform bar is supported by slopes that give them stability, fundamental con the structure rests on the walls of the hydraulic tray or on its own bases, sliding rods extend vertically upward from each supporting base, for example, from n-shaped profiles inserted into each other, supported by slopes, the upper part of the sliding rods has end pieces into which rolling bearings or other types are inserted for the platform axis, both parts of the fundamental structure are connected, for example, by bolts with detachable connections with a length corresponding to the width of the hydrohole, while the hydrohole has viewing windows and is made with NOSTA quickly change the level of flooding to provide a predetermined level of immersion in the water model during its suspension to the platform.

Figure 1 shows a possible variant of the proposed device. It includes:

- hydrowell 1 with transparent inspection windows 2 and systems for draining and filling them to the required level (not shown);

- test model 3, incorporating a device for attaching from below to the platform (not shown).

- a platform consisting of a rigid frame 4, welded from squares, sliding rods 5, made, for example, of n-shaped profiles inserted into each other, which are rigidly connected with the lower ends, for example, to the base of the platform 4, supported by slopes 6 and are tied together by jumpers 7, and the upper ones are rigidly connected to the horizontal axis 8, rolling bearings or prisms are fixed at its ends, on which the platform swings in the ends of the foundation structure, inside the frame from the squares, the gauges are stationary chnoe device consisting of calibration loads m _T, moved laterally along the rail, in addition, provides a platform for the reception of additional weights (not shown);

- a foundation structure consisting of 2 bases 11, from a channel or n-profile, resting on the walls of the hydraulic tray (or on its own foundation), 2 sliding rods 12, made of n-shaped profiles inserted into each other, welded to the bases and having ends 13 in the upper parts, into which, after assembly, the supporting bases of the prisms are inserted and fixed on the axis or rolling bearings 8, while the rods 12 are supported by slopes 13, and two parts of the base structure are bolted by jumpers 15;

- measuring block of angular oscillations of the platform 10, attached to its axis 8 and the sliding rod of the foundation.

Two parts of the foundation structure are bolted by jumpers 14.

The proposed device is assembled and calibrated as follows:

1. The foundation design is mounted: first, its supporting parts 11 with rods 12, slopes 14 and are connected by jumpers 15 are installed on the walls of the hydrohole.

2. Assigned to the lifting height of the trailer 13.

3. The lifting height of the axis 8 in the sliding rods 4 of the platform is assigned.

4. The platform assembly is weighed, its mass m _pl is determined, after which it is inserted into the end caps 13 with its bearings 9.

5. A meter [10] of the angles of its inclination is attached to the axis of the platform and is fixed on the rod 12.

6. Static calibration of the platform is performed. For this purpose, calibration weights, which are in the initial position in the middle of the platform, are moved at distances b ₁ , b ₂ , b ₃ from the middle and the bank angles Θ ₁ , Θ ₂ , Θ ₃ are fixed. The generated heeling moment is compared with the restoring one and the distance of the center of mass of the platform from the suspension axis is found for different b _i

и находится их математическое ожидание (l_G). Измерив удаление основания платформы от оси подвеса l, находят удаление y_G платформы от ее основания λ_G=l-l_G.

and their mathematical expectation (l _G ) is found. By measuring the distance of the base of the platform from the axis of the suspension l, find the distance y _{G of the} platform from its base λ _G = ll _G.

7. The moment of inertia of the platform proper relative to the axis of rotation is determined. To this end, first the calibration load is set to its initial position, at which the roll Θ = 0, then the platform is deflected and released. Its damped oscillations of the type of FIG. 2 are recorded by a measuring device.

These oscillations are identified by the differential equation of the physical pendulum

; кг м² - момент инерции платформы на воздухе относительно оси подвеса;Where

; kg m ² - the moment of inertia of the platform in air relative to the suspension axis;

- коэффициент момента сил трения платформы на воздухе;

- coefficient of the moment of friction forces of the platform in air;

- восстанавливающий момент;

- restoring moment;

Equation (1) is reduced to the standard form

- квадрат частоты собственных колебаний платформы без учета трения;Where

- the square of the natural frequency of the platform without taking into account friction;

- коэффициент относительного демпфирования.

- coefficient of relative damping.

According to the curve of damped oscillations by known methods (Reference on the theory of the ship "Statics of ships. Ship pitching" vol. 2. / Edited by Y.I. Voytkunsky. - L .: Sudostroenie, 1985, p.417) are the oscillation period and their logarithmic coefficient attenuation. Since the attenuation in air is negligible and

After completing this step, the proposed device is ready for operation. The proposed device for the purpose as follows:

1. The model is weighed and the position of its center of mass along the length and height is determined by known methods.

. Из этого выражения находится удаление от оси вращения центра масс платформы вместе с моделью и может быть сделана дополнительная оценка положения по высотке центра масс модели.2. The model is attached from below to the platform and the static calibration of the platform with the model is given in the air. In order to create a larger roll angle, in addition to the standard calibration load, an additional load can be applied in the form of an outboard hydroloader through the block on the cable for additional loads or additionally loaded on the platform ballast weights m ₀ , which after tilting remain inside the platform without creating a roll. As a result, the recovery factor of the platform can be determined together with the model

. From this expression, we find the distance from the rotation axis of the center of mass of the platform along with the model, and an additional estimate of the position along the height of the center of mass of the model can be made.

,

,

.3. The platform with the model in the air deviates at an angle | Θ | ≤15 ° and is released. Its damped oscillations are recorded and, according to the method described above, the total moment of inertia of the platform with the model and the coefficient of air resistance (damping) and friction are determined. From the point of view of saving time for conducting a series of experiments with a model in water, it is advisable to preliminarily conduct a series of experiments on rolling the platform with the model at different lengths of telescopic rods and build a relationship

,

,

.

4. The model with the help of sliding rods is installed at the level specified in the hydrohole, at which its waterline should be. After that, the hydraulic tray is filled with water to the specified level.

.5. Static calibration of the platform with the model in water is carried out and the total recovery moment from the stability of the platform load, as well as the stability of the model form from its inclination by an angle определяется when the platform is inclined, is determined

.

6. After that, the platform with the model is manually deflected by the angle отпуск and released. Its damped oscillations are measured and recorded on recording equipment, such as a computer.

These oscillations are identified by the equation

It is reduced to a form similar to (2)

- частота собственных колебаний платформы с моделью без сопротивления;Where

- natural frequency of the platform with the model without resistance;

- коэффициент относительного демпфирования всей системы.

- coefficient of relative damping of the entire system.

Using the curve of the damping process, similar to figure 2, but with a greater degree of damping according to the method described in [3], by the period τ and the decrement decrement find the frequency

Then

и

. Поскольку известно, что в жидкости и зависят от частоты и амплитуды колебаний, то колебания платформы с моделью необходимо производить на разных длинах ее подвеса и с разными балластными грузами m_б.Subtracting from these values the values that are characteristic only of a platform with a model in the air, we can find purely for the model the values

and

. Since it is known that in a liquid they depend on the frequency and amplitude of oscillations, the platform with the model must be oscillated at different lengths of its suspension and with different ballast weights m _b .

Oscillation frequency

where l _b is the removal of the center of mass of the additional ballast from the axis of the suspension;

- удаление от оси подвеса центра масс всего комплекса. Рекомендуется вариацией длин подвески платформы и балласта добиться достаточно широкого диапазона частот (периодов) колебаний модели и построить функциональную зависимость

. Вычитая из этих суммарных зависимостей аналогичные зависимости по п.3 на воздухе при одинаковых частотах получают искомые зависимости присоединенного момента

и коэффициента демпфирования

.

- removal from the suspension axis of the center of mass of the entire complex. It is recommended that by varying the suspension lengths of the platform and ballast, a sufficiently wide range of frequencies (periods) of model oscillations is achieved and a functional dependence is constructed

. Subtracting from these total dependencies similar dependencies according to claim 3 in air at the same frequencies, we obtain the desired dependences of the attached moment

and damping coefficient

.

и демпфирования N

модели, можно перейти к силам присоединенной инерции и демпфированияGiven the small curvature of the trajectory along which the model with the platform sways, from certain attached moments of inertia

and damping N

models, we can go to the forces of attached inertia and damping

где λ_{пΣ -} суммарное удаление от оси подвеса центра плавучести модели. В последующем можно уже перейти к безразмерным коэффициентам k

и r

.

where λ _{pΣ is the} total distance from the suspension axis of the center of buoyancy of the model. Subsequently, we can already pass to the dimensionless coefficients k

and r

.

The proposed device significantly expands the possibilities of the method of free damped oscillations and in relation to the known technical solutions has a number of positive advantages:

1. It removes the limitation of the method of free damped oscillations and allows you to determine the attached masses and damping coefficients at different frequencies, and not on one eigenvalue.

2. When applying the methodology [3] for processing the results, the proposed device will give results that are adequate to the devices of forced oscillations, differing from it in simplicity and low cost.

3. It has versatility, because allows you to attach different models to the platform, and at different angles, and determine the coefficients of the attached masses and damping for rolling and pitching.

, где A₀ - амплитуда волны, а λ - ее длина. Например, на 5 баллах средняя амплитуда A₀=1,4 м, а длина 45 м. Для нее α_BC=0,195 рад=11° 2.4. Allows you to simulate the angle of the wave slope in the experiment

where A ₀ is the amplitude of the wave, and λ is its length. For example, at 5 points, the average amplitude is A ₀ = 1.4 m, and the length is 45 m. For her, α _BC = 0.195 rad = 11 ° 2.

If the distance of the model (not the platform) from the suspension axis is 1.6 m, the deviation of the model from the vertical of equilibrium should be 0.31 m, the sub-float of the model will be 3 cm.

5. The flow conditions around the model under the conditions of its rolling more correspond to reality in comparison with the measurement of forces and moments on a fixed model.

All this allows us to assume that the proposed technical solution meets the criterion of a significant positive effect.

It also meets the criterion of technical feasibility, because known and non-deficient parts are used for its manufacture.

Patent search did not reveal such a technical solution. Therefore, it meets the criteria of patent novelty.

INFORMATION SOURCES

1. Guide to the theory of the ship "Statics of ships. Ship pitching ”Vol. 2 / Edited by Y.I. Voytkunsky. - L .: Shipbuilding, 1985, pp. 417-419.

2. Belyakov O.A. “Determination of the moments of inertia of large bodies by vibrations in an elastic suspension. The dissertation for the degree of candidate of physical and mathematical sciences. - M.: Moscow State University. M.V. Lomonosova, 2003.

3. Razumeenko Yu.V. The issues of transfer of hydrodynamic coefficients, determined by the methods of damped oscillations, to forced oscillations or the effect of waves on underwater technical objects. - M.: Publishing. AN "Mechanics of a solid body" No. 1, 1993.

Claims (1)

A device for determining the forces of attached inertia and damping of bodies by methods of their free damped oscillations in a liquid, including a model of a vessel or other body, a platform for providing their own vibrations around fixed supports, equipment for measuring the angle of deviation of the platform from its equilibrium position and recording its free oscillations in time after deviation and a hydroflow, characterized in that the platform is made in the form of a rigid frame, for example of metal squares, equipped with calibration weights, the model is attached at different angles from below, two strong sliding rods extending vertically from the frame, made, for example, from n-shaped profiles nested into one another, the lower ends of which are firmly connected, for example by welding, to the frame, and the upper ends are rigidly connected to axis, together with which the platform itself with calibration weights together with the model or with additionally accepted weights can oscillate in bearings at the ends of this axis, inserted into special sockets (limit switches) located in the foundation structure, each unit the platform is supported by slopes that give them stability, the fundamental structure is supported by the walls of the hydrowell or on its own bases, sliding rods extend vertically upward from each supporting base, for example, from u-shaped profiles inserted into each other, supported by slopes, the upper part of the sliding rods has the ends into which the bearings mounted on the axis of the platform are inserted, both parts of the fundamental structure are connected, for example, on bolts by detachable bonds with a length corresponding to the width gidrolotka, wherein gidrolotok has a viewing window, and configured to operatively change the level of flooding to provide a predetermined level of immersion in the water model during its suspension to the platform.

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