Abstract
Turbulence grids are widely used in wind tunnels to produce representative turbulence levels when testing aerodynamic phenomena around models. Although the purpose of the grid is to introduce a desired turbulence level in the freestream flow, the wall boundary layers of the tunnel are subjected to modification due to the presence of such grids. This could have major implications to the flow around the models to be tested and hence there is a need to further understand this interaction. The study described in this paper examines wind tunnel wall boundary layer modification by turbulence grids of different mesh sizes and porosities to understand the effect of these parameters on such interaction. Experimental results are presented in the form of pressure loss coefficients, boundary layer velocity profiles and the statistics of turbulence modification.
Highlights
Grids of many shapes, fabrication types and working principle have been extensively used by aerodynamicists to introduce representative flow perturbations and turbulence into wind tunnel test sections
The study described in this paper examines wind tunnel wall boundary layer modification by turbulence grids of different mesh sizes and porosities to understand the effect of these parameters on such interaction
A set of experiments have been conducted in a lowspeed wind tunnel with a modified cross section where an interchangeable turbulence grid could be placed
Summary
Grids of many shapes (round, square, structured, fractal, etc.), fabrication types (woven, welded/braced, perforated) and working principle (static, agitated/ pulsed, jet driven, active and passive) have been extensively used by aerodynamicists to introduce representative flow perturbations and turbulence into wind tunnel test sections. Since the turbulence produced by the grid decays with the flow downstream, the position of the grid installation becomes important In many occasions this means that the grid needs to be positioned in relative proximity to the test section and the model. An interesting issue that is less discussed when one is encountered with the latter scenario is the interaction of the wind tunnel wall boundary layer upstream of the grid with the grid structure itself Such interaction could produce unwanted localised flow variations both in an instantaneous and time averaged sense that could extend for an appreciable distance downstream and could affect the test conditions if the model to be tested is in close proximity
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