Experimental Investigation of the Flow Structure over a Delta Wing Via Flow Visualization Methods

Summary

Here, we present a protocol to observe unsteady vortical flows over a delta wing using a modified smoke flow visualization technique and investigate the mechanism responsible for the oscillations of the leading-edge vortex breakdown locations.

Abstract

It is well known that the flow field over a delta wing is dominated by a pair of counter rotating leading edge vortices (LEV). However, their mechanism is not well understood. The flow visualization technique is a promising non-intrusive method to illustrate the complex flow field spatially and temporally. A basic flow visualization setup consists of a high-powered laser and optic lenses to generate the laser sheet, a camera, a tracer particle generator, and a data processor. The wind tunnel setup, the specifications of devices involved, and the corresponding parameter settings are dependent on the flow features to be obtained.

Normal smoke wire flow visualization uses a smoke wire to demonstrate the flow streaklines. However, the performance of this method is limited by poor spatial resolution when it is conducted in a complex flow field. Therefore, an improved smoke flow visualization technique has been developed. This technique illustrates the large-scale global LEV flow field and the small-scale shear layer flow structure at the same time, providing a valuable reference for later detailed particle image velocimetry (PIV) measurement.

In this paper, the application of the improved smoke flow visualization and PIV measurement to study the unsteady flow phenomena over a delta wing is demonstrated. The procedure and cautions for conducting the experiment are listed, including wind tunnel setup, data acquisition, and data processing. The representative results show that these two flow visualization methods are effective techniques for investigating the three-dimensional flow field qualitatively and quantitatively.

Introduction

Flow field measurement via visualization techniques is a basic methodology in fluid engineering. Among the different visualization techniques, smoke wire flow visualization in wind tunnel experiments and dye visualization in water tunnel experiments are the most widely used to illustrate flow structures qualitatively. PIV and laser Doppler anemometry (LDA) are two typical quantitative techniques¹.

In smoke wire flow visualization, smoke streaklines are generated from oil droplets on a heating wire or injected from the outer smoke generator/container during the experiments. High-power lights or laser sheets are used t....

Protocol

1. Wind Tunnel Setup

1. Delta wing model
   1. Construct a delta wing model from aluminum, with a sweep angle φ of 75°, a chord length c of 280 mm, a root span b of 150 mm, and a thickness of 5 mm. Have both leading edges beveled at 35° to fix the separation point¹⁷ (see Figure 1a).
2. Wind tunnel facility
   1. Conduct experiments in a closed-loop low-speed wind tunnel, with a test section of 2.4 m (length) × 0.6 m (width) × 0.6 m (height) that is equipped with glass walls that allow optical access during the exp....

Results

Figure 2d shows the time histories of the LEV breakdown locations. The black curve indicates the portside LEV and the red curve indicates the starboard LEV. The time scale is nondimensionalized by the free stream velocity and chord length. The correlation coefficient between these two time histories is r = −0.53, indicating a strong anti-symmetric interaction of the LEV breakdown location oscillations. This result agrees well with the work of others<.......

Discussion

This article presents the two flow visualization methods, improved smoke flow visualization and PIV measurement, to investigate flow structure over the delta wing qualitatively and quantitatively. The general procedures of the experiment are described step by step. The setups of these two methods are almost the same, while the devices involved are different. The basic principle of these two flow visualization methods is to illuminate the particles in the flow via the laser sheet. The improved smoke flow visualization can.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
532 nm Nd:YAG laser	Quantel Laser	Evergreen 600mJ
High speed camera	Dantec Dynamic	HiSense 4M
camera lens	Tamron	SP AF180mm F/3.5 Di
PIV recording and processing software	Dantec Dynamic	DynamicStudio
cylindrical lens	Newport		Φ=12 mm
convex lens	Newport		f=700 mm
neutral density filter	Newport
Calibration target			custom made
aerosol generator	TSI	TSI 9307-6
PULSE GENERATOR	Berkeley Nucleonics Corp	BNC 575
continuous laser		APGL-FN-532-1W
Digital camera	Nikon	Nikon D5200
Image processing	Matlab		custom code
wind tunnel support			custom made
laser level	BOSCH	GLL3-15X
angle meter	BOSCH	GAM220