Field measurements of turbulence at an unstable interface between current and wave bottom boundary layers

Abstract

[1] In situ particle image velocimetry measurements, at a resolution of 3.5 Kolmogorov scales, have been performed in the inner part of the coastal bottom boundary layer. The spatial details enable us to directly determine the vertical distributions of mean velocity, Reynolds shear stress, shear production and dissipation rates, energy spectra, and abundance of eddies. Focusing on cases with wave velocity of similar magnitude as the mean current, velocity profiles have logarithmic distributions in the upper half of the sample area. Below the log layer, but well above the bottom ripples, an inflection point appears, indicating a region of flow instability. Based on data interpretation, which includes variations in wave phase with height, this inflection occurs near the interface between current and thinner wave boundary layer (WBL) below it. Scaling of mean velocity profiles with shear velocity and characteristic roughness is effective only above the inflection point, while turbulence parameters scale reasonably well at all elevations. Instabilities associated with the inflection are manifested by a peak in turbulent shear production rate and a rapid increase in small-scale turbulence, as is evident from trends of the dissipation rate, energy spectra, and distribution of eddies with elevation. Therefore, the presence of a WBL generates a shear production peak and rapid increase in the dissipation rate at higher elevations than those found in rough-wall steady boundary layers. Transition between current and wave boundary layers is also characterized by broad Reynolds stress peaks and shear production exceeding the dissipation rate.Citation: Hackett, E. E., L. Luznik, A. R. Nayak, J. Katz, and T. R. Osborn (2011), Field measurements of turbulence at an unstable interface between current and wave bottom boundary layers,

Near-wall turbulence statistics and flow structures over three-dimensional roughness in a turbulent channel flow

“…In Nimmo Smith, Katz & Osborn (2005), we verified that these bumps are not an artefact of the measurement or analysis procedures by applying the same codes (PIV analysis and spectral calculations) to process laboratory isotropic turbulence data and confirming that they do not have bumps. In support of our claims, Hackett et al (2010) show that the length scale associated with the oceanic bumps, ∼ 2 cm, is very different from the present scales, but matches the local height of bottom ripples. Second, spectral bumps have been observed in a few canopy flow studies (e.g.…”

“…within the dissipation range. A very similar trend can be seen in the oceanic data of Hackett et al (2010), where the spectral bumps appear in wavenumbers corresponding to ∼ 1.6k-2.4k or 13η-20η, but in this case k ∼ 1 cm and η = 1.2 mm. In both cases, these bumps appear at higher k 1 η, i.e.…”

On the wave and current interaction with a rippled seabed in the coastal ocean bottom boundary layer

“…For ensemble‐averaged quantities, the uncertainty caused by random errors is reduced by nearly 2 orders of magnitude. However, one should keep in mind that the mean velocity varies over time during each run, as demonstrated in prior studies [ Hackett et al ., ]. For instantaneous velocity gradients, the uncertainty can be an order of magnitude higher, i.e., about 20%, ignoring effects of spatial filtering by the finite resolution.…”

“…Such measurements have been performed using profiling thermistors [ Caldwell and Chriss , ; Chriss and Caldwell , ], hot film anemometer arrays [ Foster et al ., , 2006], laser Doppler velocimeters [ Agrawal and Aubrey , ; Trowbridge and Agrawal , ], acoustic Doppler velocimeters [ Davis and Monismith , ; Feddersen et al ., ; Walter et al ., ], acoustic current meters [ Trowbridge , ; Williams et al ., ], and coherent Doppler profilers [ Lacy et al ., ; Hay , ; Zou et al ., ]. Our group has introduced the application of 2‐D particle image velocimetry (PIV) to resolve the flow in the inner part of the BBL, achieving 4–5 mm resolution over sample areas ranging from 20 × 20 cm 2 to 50 × 50 cm 2 [ Bertuccioli et al ., ; Doron et al ., ; Hackett et al ., ; Luznik et al ., ; Nimmo Smith et al ., , 2005]. Other PIV systems have been deployed recently to study turbulence structure in oceanic and fluvial environments [ Liao et al ., ; Steinbuck et al ., ; Tritico et al ., ].…”

A Free-Floating PIV System: Measurements of Small-Scale Turbulence under the Wind Wave Surface

“…Near-surface coherent structures can be resolved in the vertical 2D velocity map derived from PIV measurements. Nevertheless, a partially submersible PIV system has been developed and successfully deployed to study turbulent flow structures in coastal oceans during the past decade (Doron et al 2001;Nimmo Smith et al 2002;Hackett et al 2011). In general, standard PIV systems are unsuitable for field deployments because of the complex optical configurations and high demands on electrical power and computing resources.…”