We have designed, fabricated, and field-tested a new, unique, monostatic, broadband, electromagnetic sensor for subsurface geophysical investigation. The sensing unit consists of a pair of concentric, circular coils that transmit a continuous, broadband, digitally-controlled, electromagnetic waveform. The two transmitter coils, with precisely computed dimensions and placement, create a zone of magnetic cavity (viz., an area with a vanishing primary magnetic flux) at the center of the two coils. A third receiving coil is placed within this magnetic cavity so that it senses only the weak, secondary field returned from the earth and buried targets.This monostatic configuration has many advantages including (1) compact sensor head, (2) a large transmitter moment, (3) high spatial resolution, (4) no spatial distortion of an anomaly common to bistatic sensors, (5) circular symmetry that greatly simplifies mathematical description, and, therefore, (6) simplified forward and inverse modeling processes. Three prototype GEM-3 units have been built and tested at various environmental sites, including those containing unexploded ordnance and land mines.
Additively manufactured surfaces that are pointed upward have been shown to exhibit roughness characteristics different from those seen on surfaces that point downward. For this investigation, the Roughness Internal Flow Tunnel (RIFT) and computational fluid dynamics models were used to investigate flow in channels with different roughness on opposing walls of the channel. Three rough surfaces were employed for the investigation. Two of the surfaces were created using scaled, structured-light scans of the upskin and downskin surfaces of an Inconel 718 component which was created at a 45-deg angle to the printing surface. A third rough surface was created for the RIFT investigation using a structured-light scan of a surface similar to the Inconel 718 downskin surface, but a different scaling was used to provide larger roughness elements in the RIFT. The resulting roughness dimensions (Rq/Dh) of the three surfaces used were 0.0064, 0.0156, and 0.0405. The friction coefficients were measured over the range of 10,000 < ReDh < 70,000 for each surface opposed by a smooth wall and opposed by each of the other rough walls. X-array hot film anemometry was used to characterize the velocity and turbulence profiles for each roughness combination. The results of inner variable analysis demonstrate that the roughness function (ΔU+) becomes independent of the roughness condition of the opposing wall providing evidence that Townsend's Hypothesis holds for the relative roughness values expected for additively manufactured turbine-blade cooling passages.
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