SU964282A1 - Method of measuring tangential stress of rough surface being flowed around by turbulent flow - Google Patents

Description

(5) METHOD OF MEASURING TENSIONAL STRESS ON A ROUGH SURFACE FLOORED

  V.

The invention relates to hydraulics and aerodynamics, including both. internal and external fluid flow around the body of solids.

A known method for measuring the tangential stress on a rough surface flowed by a turbulent flow is measured by measuring the average longitudinal velocity component in the near-wall part of the layer and the distance to the wall and constructing an average component profile of the longitudinal velocity network 1.

A disadvantage of the known method is the low accuracy of measuring the tangential stress on a rough surface in a turbulent boundary layer.

The purpose of the invention is to improve accuracy.

This goal is achieved by the additional measurement of the mean square pulsation TURBULENT FLOW

the longitudinal velocity component and determine the distance from the wall to the surface with the maximum rms pulsation component, and the tangential stress is determined from the ratio

.tn (

(one)

ten

where l is the density of the liquid;

9 - kinematic coefficient

15 viscosity;

Yg is the distance from the wall to the surface of the maximum of the pulsation profile;

Ug - the value of the average longitudinal

20 velocity at a distance Yg from the wall;

C is tangential stress. The use of the proposed method is due to the fact that the surface with the maximum value of the mean square pulsation component of the longitudinal velocity corresponds to a certain distance from the ultrasonic wall, and this distance from the wall Yg is uniquely related by the relation (1) to the average longitudinal velocity and disagreement. Since the fluid density o and the kinematic viscosity coefficient 9 are known before measuring the tangential stress on the wall Cw, after setting the values of Yg and Ug in expression (1), only the value is unknown. which is determined from (1) the fit method. The surfaces with the maximum value of the mean square pulsation component of the longitudinal velocity COOT correspond to the kink of the profile of the average longitudinal velocity. Therefore, the value of the average longitudinal velocity Ug can also be determined by the fracture of its profile. The tangential stress on a rough surface is measured as follows. Know in advance (or determine) the fluid density p and the kinematic viscosity coefficient -P. The gauge of the mean square ripple of the longitudinal velocity and the mean longitudinal velocity associated with the coordinate axis moves in the near-wall part of the turbulent boundary layer and measures the mean square pulsation of the longitudinal velocity, the readout of the recorder or in the computer. The maximum value of the mean square pulsation of the longitudinal velocity and the distance Yg from the maximum to the wall are determined. The value and mean longitudinal velocity is measured at a distance from the wall Yg. The values of Uc, Ye, 1, and p are substituted into relation (1) and the tangential stress on the wall is determined by the selection method, for example, according to an appropriate program in a computer. Moreover, it is possible to simultaneously measure the mean square pulsation of the longitudinal velocity and the average prodonal velocity with the same sensor, for example, a thermal anemometer, which simultaneously measures both these quantities, or the measurement of these quantities with different sensors successively in time. It is also possible to measure only the longitudinal velocity in the wall part of the layer, determine the kink of the velocity profile and the distance to Him from the wall Yg. readout from the reduced surface (given diameter for the pipe) and counting from the inner surface (internal diameter for the pipe) For most types of rough surface, which are characterized by a significant contribution of the vortex O zones formed on the roughened surface, resulting in turbulent flow, the distance from the wall was measured by the above surface. The surface shown is determined experimentally by pouring a rough surface with water to determine the amount of water that filled the trough. In the case of a rough pipe, the reduced diameter is D, pH4V / ftL, where V is the internal volume of the pipe section, LI is its length. The value of volume V is judged by the volume of water that filled it. Since the pipe axis is uniquely determined, the distance Y is measured from the surface located from the pipe axis at a distance equal to Df, p / 2. In the case of a rough surface on the plate, it is covered with water immediately before the protrusions are fully immersed. Then the volume of water is V: HLtnp, (3) where L is the length of the plate; H is the width of the plate. From (3), t is determined from which Y is read. With a rough rough surface, when due to the relatively large depth of the depressions, the vortex zones formed in the depressions make a small contribution to the resulting fluid flow in the main flow direction, Y is measured from the internal diameter in the case of a rough flow.

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pipe or from the tops of the projections in the case of flow on a flat plate.

Using the proposed method of indirectly measuring the tangential stress on a rough surface in a turbulent boundary layer provides an increase in the accuracy of the measurement of the tangential stress.

Claims (1)

Invention Formula A method for measuring tangential stress on a rough surface flowed by a turbulent flow by measuring the average longitudinal velocity component in the wall section and the distance to the wall and constructing an average longitudinal velocity profile, characterized in that, in order to improve accuracy, measure the rms pulsation component of the longitudinal velocity and determine the distance from the wall to the surface with the maximum rms pulsation component, and the tangential stress determines from the ratio R. de fluid density; kinematic viscosity coefficient; distance from the wall to the surface of the maximum profile of the pulsation;  - the value of the average longitudinal velocity at a distance Yg from the wall;  - tangential stress. Sources of information taken into account in the examination 1. Roth I, K. Turbulent boundary layer of incompressible fluid. about. | L. Shipbuilding 1967, p. 178.