Abstract

The generation of a thick fully turbulent boundary layer is investigated in a lowspeed wind tunnel at a nominal zero pressure gradient over the Reynolds number range 0.145× 10 6 · Rex · 0.58× 10 6 . The wind tunnel floor natural boundary layer is laminar with thickness ± between 5.76 mm and 8.13 mm. Different tripping devices are tested to trigger transition so to double the boundary layer thickness and provide a fully established turbulent velocity profile. Using a trip wire significantly increases ± but leads to an unsatisfactory velocity profile. Using a sandpaper strip slightly increases ± but keeps the boundary layer laminar. Using a strip of sharp-edged silicon granules doubles the boundary layer thickness that increases up to 20 mm and the mean velocity profiles are a good fit to the logarithmic law of the wall over the outer region of the boundary layer. The spectral decay of turbulent kinetic energy in this outer layer is exponential and close to i5/3, indicating turbulence equilibrium. This work is of practical interest to wind tunnel practitioners for generating equilibrium thick turbulent boundary layers at low Reynolds numbers.

Highlights

  • This paper contributes to the literature by investigating by experiment boundary layer tripping at low Reynolds numbers, with the dual objective of achieving a fully turbulent boundary layer, with well-developed turbulent statistics, and a pre-defined thickness, at a given location downstream of the trip

  • The laminar state of the boundary layer is confirmed by the shape factor H > 2.5 and the absence of a logarithmic growth in the normalized velocity profile

  • Applying a trip wire gives a significant growth in boundary layer thickness

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Summary

Introduction

Scaled up models are commonly used in wind tunnels to obtain spatially resolved measurements from geometries that are difficult to access, such as that of an automobile door seal cavity. To match the inflow Reynolds number at full-scale, these tests often require the generation of a fully turbulent inflow boundary layer at low speeds, the thickness of which is of the order of 20 mm. To match the inflow Reynolds number at full-scale, these tests often require the generation of a fully turbulent inflow boundary layer at low speeds, the thickness of which is of the order of 20 mm. To overcome these conflicting requirements, it is common experimental practice to use a boundary layer leading edge trip This allows to trigger the boundary layer transition to turbulence at a relatively low Reynolds number while increasing the boundary layer thickness by the thickness of the wake downstream of the trip. This paper wishes to be a useful aid to experimental aerodynamicists for selecting a boundary layer trip device in an application that requires a low velocity thick turbulent boundary layer inflow

Description of the experimental setup
Test conditions
Effect of the tripping devices
Flow analysis
Boundary layer trip device selection
Findings
Concluding remarks
Full Text
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