Trans RINA, Vol 152, Part B1, Intl J Small Craft Tech, 2010 Jan-Jun 3.1 (a) Traditional Instrumentation


Whilst pitot probes are perhaps the simplest to use and manufacture, the tubing connecting the probe to the transducer can, if too long, aggressively low pass filter the results.


Therefore, the temporal resolution of the


measurement may be limited. Whilst the characteristic timescale of large eddies (i.e. those which may influence rotorcraft) may be sufficiently long to resolve using pitot probes, under


scale conditions the equivalent eddy


timescales will be significantly smaller and higher temporal resolution techniques may be required.


Hotwires resolve the temporal resolution limitation of pitot probes by determining velocity fluctuations from the heat transfer to very fine wire held in the flow. There is a substantial amount of


required, and the wires are fragile and prone to breakage.


Mean velocity results may also be useful to indicate how the superstructure influences the behaviour of plumes and the dispersion of exhaust gases. Mean velocity results may be derived by time-averaging signals derived from point probes.


A limitation of probe based techniques is inherent point- by-point


(or pointwise) determination of nature of the measurement


process. For a large model, a significant amount of time will be required to build up a detailed map of each flow variable throughout the domain. This is particularly true for the


standard deviation of vertical velocity) where the data must be sampled for a considerable period at sufficient temporal resolution in order to acquire a robust result.


Additionally, the presence of the probe has the


undesirable effect of disturbing local flow features in the airwake. Therefore probe techniques are termed Invasive. For large scale experiments the invasive nature of the measurement may be reduced by using small streamlined probes. However the effects may be more notable during scale testing and where turbulent fluctuations are important as is the case for airwake measurements.


A fundamental limitation of pointwise measurement is that the results at each point are essentially independent of each other.


Therefore the results may only provide


limited information regarding the interaction between spatial flow structures, for example the behaviour of large vortices in the wake of the superstructure.


3.1 (b) Advanced Instrumentation


Following the development of the first laser in 1960, the potential


for using these devices for non-invasive


aerodynamic measurement was quickly realised. The first demonstration of the use of lasers in experimental aerodynamics was reported in 1964 using an interferometric technique


termed Laser Doppler statistical parameters (i.e.


Velocimetry, LDV [6]. This technique is widely in use today and enables capture of high temporal resolution velocity data at a point marked by the intersection of two laser beams.


Despite the non-invasive nature of the LDV technique, its major limitation is the pointwise measurement approach which leads to considerable effort to map a large flow domain. For example, some researchers have reported the use of LDV to map the turbulent fluctuations in the wake of simplified car models [7] and above the helicopter landing pad of a Type 23 Frigates


[8]. calibration of equipment


Typically 5 minutes of data acquisition was required at each point to produce robust turbulence estimates and many weeks of test time was required to map the wake at a single flow condition. Whilst much of this analysis could be undertaken by computer controlled traversing equipment, it is still a considerable undertaking. Similar overall


timescales would be expected using other


invasive probe based techniques (i.e. pitot probes or hotwires) to achieve equivalent spatial resolution of the wake.


To address the pointwise limitations of LDV a number of planar laser velocimetry techniques have been developed over the past thirty years. Such techniques are termed Wholefield due to their ability to simultaneously capture data at multiple points within the flow domain. At the present time the most practical planar Particle Image Velocimetry (PIV) [9]. motion of


technique


the flow using a pulsed laser. simultaneous measurement


is PIV tracks the


measurement plane between two successive illuminations of


seeding particle groups within the The result is a


of in-plane velocity


components (i.e. vertical and transverse) with good temporal and spatial resolution. PIV systems are commercially available and are commonly found in advanced wind tunnel facilities.


A recent and promising frequency based technique


termed Doppler Global Velocimetry (DGV) was first reported in 1991 [10]. Unlike PIV, DGV requires only a single illumination from a laser pulse and therefore offers potentially higher spatial and temporal resolution. As the operation of the technique is based in sensing the Doppler shift in scattered light frequency induced by the flow motion,


the technique is particularly robust at


measuring strong out of plane motions. The result is that the three-components of velocity (x, y and z) can be captured simultaneously measurement plane.


within a two-dimensional DGV systems are currently not


readily commercially available and must be developed from scratch. Hence this technique currently remains an academic tool. However, recent advances in the design of DGV systems have significantly reduced the cost and complexity of equipment such that these systems may become common place at industrial wind tunnels in the near future [11].


Figure 2 shows a schematic of the


complex optical bench arrangements which are typically required for DGV and other laser based techniques.


B-24


©2010: The Royal Institution of Naval Architects


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