Experimental investigation on turbulence intensity reduction in subsonic wind tunnels

The Importance of Turbulence Reduction in Assessment of Wind Tunnel Flow Quality

“…Then the effective velocities will determined and translate to the wire-coordinate system. As a result, the velocities component in the probe coordinate system, U and V will determine [7]. Subsequently, one may obtain the time averaged velocity, Umean and Vmean, of the flow at a particular point by taking the average of the instantaneous velocity U and V. By taking the difference of the time averaged velocity and the instantaneous velocity, the instantaneous fluctuating velocity u and v is obtained.…”

“…If separation occurs, then the flow uniformity and steadiness will be degraded which may lead to an increase in turbulence intensity in the test section. In summary, contractions in the wind tunnels may produce several different unsteady secondary flows which are undesirable and can have dramatic effects on the behavior of the downstream boundary layers and turbulence intensity in test section [7,23]. The boundary layer flow over a surface with a region of concave curvature is susceptible to centrifugal instabilities in the form of Gortler vortices [23].…”

“…Nevertheless, honeycombs also shed turbulence, the strength of which is proportional to the shear layer thickness in the cells. Therefore, honeycomb is supposed to break the large eddies into small ones, thus a deep honeycomb performs better than a shallow one but the pressure loss across it is larger [7]. Furthermore, the choice of appropriate screens is also difficult.…”

“…Screens with smaller porosity suffer from a flow instability that appears in the test section. Whether screens or honeycombs, the obtained reduction in the free stream turbulence level is accompanied with a power loss due to manipulator pressure drop and hence reducing the maximum attainable velocity in the test section of the wind tunnel [7]. Fig.…”

“…The sensitivity is then given by the slope of the velocity-voltage relationship and curve-fitting is commonly used to reduce the error in differentiating the discrete data. Polynomials as well as curves based on the King's law have been used by various researchers [7][8][9][10]. In a 2-D crossed wire probe the sensitivity may be found by a method similar to that used in calibrating a normal wire.…”