Stephen M. Walker

DOE's Effort to Reduce Truck Aerodynamic Drag Through Joint Experiments and Computations

Class 8 tractor-trailers are responsible for 11-12% of the total US consumption of petroleum. Overcoming aero drag represents 65% of energy expenditure at highway speeds. Most of the drag results from pressure differences and reducing highway speeds is very effective. The goal is to reduce aerodynamic drag by 25% which would translate to 12% improved fuel economy or 4,200 million gal/year. Objectives are: (1) In support of DOEs mission, provide guidance to industry in the reduction of aerodynamic drag; (2) To shorten and improve design process, establish a database of experimental, computational, and conceptual design information; (3) Demonstrate new drag-reduction techniques; and (4) Get devices on the road. Some accomplishments are: (1) Concepts developed/tested that exceeded 25% drag reduction goal; (2) Insight and guidelines for drag reduction provided to industry through computations and experiments; (3) Joined with industry in getting devices on the road and providing design concepts through virtual modeling and testing; and (4) International recognition achieved through open documentation and database.

Two-axes Scheimpflug focusing for particle image velocimetry

The objective of this work is to demonstrate the capabilities of a two-axes, three-dimensional particle image velocimetry (PIV) system for future use as a large-scale wind tunnel research tool. A two-axes PIV system provides the capability for camera placement virtually anywhere within three-dimensional space relative to the laser sheet plane of measurement. The Scheimpflug focusing method is applied in two orthogonal axes to achieve off-axis focusing. The two-axes, three-dimensional PIV system uses off-axis stereoscopic viewing to determine two perspective measurements of a three-dimensional velocity field within the laser sheet. A unique calibration procedure is used to determine the optical parameters that describe the position of the camera systems and which are necessary to perform the three-dimensional reconstruction of the displacements. This paper presents the calibration procedure as well as a method for reconstructing the three-dimensional particle displacements for this two-axes, three-dimensional PIV system. This two-axes, three-dimensional PIV system was demonstrated by measuring three-dimensional displacements of a target. Maximum instantaneous errors are estimated as 2% or less for all velocity components. These error estimates are for instantaneous displacements and include all error sources. The two-axes, three-dimensional PIV system was also used to make measurements of the freestream velocity within a wind tunnel. The out-of-plane freestream component of velocity measured by the PIV system differed from the expected value by less than 1.1{%}.

Planar Velocimetry of Jet/Fin Interaction on a Full-Scale Flight Vehicle Configuration

Stereoscopic particle image velocimetry has been implemented in a production-scale transonic wind tunnel for studying jet/fin interaction created by exhaust plumes from spin rockets on a full-scale model of a finned body of revolution. Data acquired just upstream of the leading edge of the fin root clearly display the counter-rotating vortex pair that dominates the interaction far field and the remnant of the horseshoe vortex near the vehicle surface. The counter-rotating vortex pair is distinctly asymmetric due to originating from a scarfed nozzle and displays some rotation with respect to the model surface. Velocity fields measured over a range of flowfield conditions and model orientations show that the vortex of negative sign is always closer to the fins than its positive counterpart and does not greatly change location as flowfield parameters are altered. The circulation of this vortex correlates with a reduction in the simultaneously measured vehicle roll torque. Further correlations are hindered by untreatable bias errors in the velocimetry. Instead, a model of the vortex structure derived from the velocimetry data reveals that the angle of attack induced upon the fins by the counter-rotating vortex pair correlates with the roll torque loss. Similar correlations suggest that in level flight this effect is dominant, but at angle of attack the horseshoe vortex on the windward side has an additional influence.

Predicting Camera Views for Image-Based Measurements in Wind Tunnels

This paper describes a method for creating virtual images of wind-tunnel scenes and shows how these images can be used to plan image-based measurements. The method has been implemented as a stand-alone Windows application and was used extensively to plan Particle Image Velocimetry (PIV) measurements for recent wind-tunnel tests of a 3% scale model of the Space Shuttle. Real and virtual images from these tests are presented to demonstrate the methodology.

The Measurement of Wake and Gap Flows of the Generic Conventional Truck Model (GCM) using Three-Component PIV

Particle Image Velocimetry (PIV) measurements were acquired in the wake of the trailer and in the gap between the tractor and trailer of the Generic Conventional Model (GCM) truck for the US Department of Energy. The data will be used both for validation of computational fluid dynamics (CFD) codes and for understanding the flow physics. The GCM is a 1/8th-scale, moderate-fidelity model of a full-scale truck. The test was performed in the Army/NASA 7 × 10 wind tunnel at NASA Ames Research Center. Surface pressure and force measurements were made prior to the PIV measurements. PIV measurements were made at two yaw angles and at three horizontal planes for three model configurations, each at a free-stream velocity of 52 m/s (Mach 0.15), which corresponds to a Reynolds number of 1 × 106, based on the width of the tractor. This paper discusses the PIV system, samples of flow data and some of the observed features that may have contributed to the measured drag.

Stereoscopic PIV for Jet/Fin interaction measurements on a full-scale flight vehicle configuration.

A stereoscopic particle image velocimeter has been implemented in a production-scale transonic wind tunnel for studying jet/fin interaction created by exhaust plumes from spin rockets on a full-scale model of a finned body of revolution. Data were acquired principally at a measurement plane just upstream of the leading edge of the fin root, which clearly display the counter-rotating vortex pair as well as a smaller vortex pair near the surface believed to be the remnant of the horseshoe vortex. The counter-rotating vortex pair is distinctly asymmetric as a result of having been produced by a jet exiting from a scarfed nozzle and displays some twist with respect to the model surface. The horseshoe vortex is displaced laterally from the nozzle position. Velocities measured over a range of flowfield conditions and model orientations show that the size and position of the vortices chiefly scale with the jet-to-freestream dynamic pressure ratio and that their strength is additionally a function of the crossflow Mach number.

DOE Project on Heavy Vehicle Aerodynamic Drag FY 2005 Annual Report

Class 8 tractor-trailers consume 11-12% of the total US petroleum use. At high way speeds, 65% of the energy expenditure for a Class 8 truck is in overcoming aerodynamic drag. The project objective is to improve fuel economy of Class 8 tractor-trailers by providing guidance on methods of reducing drag by at least 25%. A 25% reduction in drag would present a 12% improvement in fuel economy at highway speeds, equivalent to about 130 midsize tanker ships per year. Specific goals include: (1) Provide guidance to industry in the reduction of aerodynamic drag of heavy truck vehicles; and (2) Establish a database of experimental, computational, and conceptual design information, and demonstrate the potential of new drag-reduction devices.

The development of a 3C-PIV system for the 12-foot pressure tunnel at NASA Ames Research Center

A three-component particle image velocimetry (3C-PIV) system was designed installed in the 12-foot (3.7 m) pressure tunnel at NASA Ames Research Center. The system was designed for the testing of a 1/8/sup th/ scale tractor-trailer model called the generic conventional model (GCM). The 12-foot PWT can be pressurized to 6 atmospheres to enable the testing of Reynolds number effects. The data requirements forced a complex system design. Furthermore, the system had to reside inside the plenum where components sensitive to high pressure had to be isolated. Data were obtained in the gap between the tractor and the trailer where Reynolds number effects were suspected to occur. Sample data from this experiment are presented in this paper.

Flow Field Measurements in the Cell Culture Unit

Abstract: The cell culture unit (CCU) is being designed to support cell growth for long‐duration life science experiments on the International Space Station (ISS). The CCU is a perfused loop system that provides a fluid environment for controlled cell growth experiments within cell specimen chambers (CSCs), and is intended to accommodate diverse cell specimen types. Many of the functional requirements depend on the fluid flow field within the CSC (e.g., feeding and gas management). A design goal of the CCU is to match, within experimental limits, all environmental conditions, other than the effects of gravity on the cells, whether the hardware is in microgravity (μg), normal Earth gravity, or up to 2g on the ISS centrifuge. In order to achieve this goal, two steps are being taken. The first step is to characterize the environmental conditions of current 1g cell biology experiments being performed in laboratories using ground‐based hardware. The second step is to ensure that the design of the CCU allows the fluid flow conditions found in 1g to be replicated from microgravity up to 2g. The techniques that are being used to take these steps include flow visualization, particle image velocimetry (PIV), and computational fluid dynamics (CFD). Flow visualization using the injection of dye has been used to gain a global perspective of the characteristics of the CSC flow field. To characterize laboratory cell culture conditions, PIV is being used to determine the flow field parameters of cell suspension cultures grown in Erlenmeyer flasks on orbital shakers. These measured parameters will be compared to PIV measurements in the CSCs to ensure that the flow field that cells encounter in CSCs is within the bounds determined for typical laboratory experiments. Using CFD, a detailed simulation is being developed to predict the flow field within the CSC for a wide variety of flow conditions, including microgravity environments. Results from all these measurements and analyses of the CSC flow environment are presented and discussed. The final configuration of the CSC employs magnetic stir bars with angled paddles to achieve the necessary flow requirements within the CSC.

Two-axis Scheimpflug focusing for particle image velocimetry

The objective of this work is to demonstrate the capabilities of a two-axis, three-dimensional Particle Image Velocimetry (PIV) system for use as a largescale wind tunnel research tool. A two-axis PIV system provides the capability for camera placement virtually anywhere within a three-dimensional space relative to the laser sheet plane of measurement. The Scheimpflug focusing method is applied in two orthogonal axes to achieve off-axis focusing. The two-axis, three-dimensional PIV system uses off-axis stereoscopic viewing to determine two perspective measurements of a three-dimensional velocity field within the laser sheet. A unique calibration procedure is used to determine the optical parameters that describe the position of the camera systems and which are necessary to perform the three-dimensional reconstruction of the displacements. This paper presents this unique calibration for a two-axis, three-dimensional PIV system. This two-axis, three-dimensional PIV system was demonstrated first by measuring three-dimensional displacements of a target. Maximum instantaneous errors are estimated as 2% or less for all velocity components based upon the measured target displacements, and these error estimates were verified by a propagation of errors calculation. The error estimates are for instantaneous displacements and include all error sources. The two-axis, three-dimensional PIV system was also used to make measurements of the freestream velocity within a wind tunnel. The out-of-plane freestream component of velocity measured by the PIV system differed from the expected value by less than 1.1 %.