The anisotropy of the slip length and its effect on the skin-friction drag are numerically investigated for a turbulent channel flow with an idealized superhydrophobic surface having an air layer, where the idealized air-water interface is flat and does not contain the surface-tension effect. Inside the air layer, both the shear-driven flow and recirculating flow with zero net mass flow rate are considered. With increasing air-layer thickness, the slip length, slip velocity and percentage of drag reduction increase. It is shown that the slip length is independent of the water flow and depends only on the air-layer geometry. The amount of drag reduction obtained is in between those by the empirical formulae from the streamwise slip only and isotropic slip, indicating that the present air-water interface generates an anisotropic slip, and the streamwise slip length (b x ) is larger than the spanwise one (b z ). From the joint probability density function of the slip velocities and velocity gradients at the interface, we confirm the anisotropy of the slip lengths and obtain their relative magnitude (b x /b z = 4) for the present idealized superhydrophobic surface. It is also shown that the Navier slip model is valid only in the mean sense, and it is generally not applicable to fluctuating quantities.

We perform direct numerical simulations of a turbulent channel flow with a lubricated micro-grooved surface to investigate the effects of this surface on the slip characteristics at the interface and the friction drag. The interface between water and lubricant is assumed to be flat, i.e. the surface-tension effect is neglected. The solid substrate, where a lubricant is infused, is composed of straight longitudinal grooves. The flow rate of water inside the channel is maintained constant, and a lubricant layer under the interface is shear driven by the turbulent water flow above. A turbulent channel flow with a superhydrophobic (i.e. air-lubricated) surface having the same solid substrate configuration is also simulated for comparison. The results show that the drag reduction with the liquid-infused surface highly depends on the lubricant viscosity as well as the groove width and aspect ratio. The amounts of drag reduction with the liquid-infused surfaces are not as good as those with superhydrophobic surfaces, but are still meaningfully large. For instance, the maximum drag reduction by the heptane-infused surface is approximately 13 % for a rectangular groove whose spanwise width and depth in wall units are 12 and 14.4, respectively, whereas a superhydrophobic surface with the same geometry results in a drag reduction of 21 %. The mean slip length normalized by the viscosity ratio and groove depth depends on the groove aspect ratio. The ratio of fluctuating spanwise slip length to the streamwise one is between 0.25 (ideal surface without groove structures) and 1 (i.e. isotropic slip), indicating that the slip is anisotropic. Using the Stokes flow assumption, the effective streamwise and spanwise slip lengths are expressed as a function of groove geometric parameters and lubricant viscosity. We also suggest a predictive model for drag reduction with the heptane-lubricated surface by combining the predicted effective slip lengths with the drag reduction formula used for riblets (Luchini et al., J. Fluid Mech., vol. 228, 1991, pp. 87–109). The predicted drag reductions are in good agreements with those from the present and previous direct numerical simulations.

No abstract

A new era of neurosurgery has recently been unveiled with the advent of image-guided surgery. The use of neuronavigation is beginning to have a significant impact on a variety of intracranial procedures. Herein, we report our clinical experience using a neuronavigation system with different surgical applications and techniques for a variety of brain tumors. We used the BrainLab VectorVision neuronavigation system, which is a frameless and image-guided system. We operated on 420 cases having various types of brain tumor with the help of this system. The mean target localizing accuracy and mean volume were 1.15 mm and 30.8 mL (0.2-216.4 mL), respectively. We utilized this system to effectively make bone flaps, to detect critically located, deep-seated, subcortical, skull-base and skull bone tumors, and to operate on intraparenchymal lesions with grossly unclear margins, such as gliomas. We also performed tumor biopsy using the combination of a conventional stereotactic biopsy instrument and an endoscope. The application of the neuronavigation system not only revealed benefits for operative planning, appreciation of anatomy, lesion location and the safety of surgery, but also greatly enhanced surgical confidence.

Objectives: Leiomyosarcoma is a malignant neoplasm that affects smooth muscle tissue and it is very rare in the field of oral and maxillofcial surgery. The purpose of this study was to obtain information on diagnosis of and treatment methods for leiomyosarcoma by retrospectively reviewing of the cases. Patients and Methods: The study included nine patients who were diagnosed with leiomyosarcoma in the Department of Oral and Maxillofacial Surgery at Seoul National University Dental Hospital. The subjects were analyzed with respect to sex, age, clinical features, primary site of disease, treatment method, recurrence, and metastasis. Results: Particular clinical features included pain, edema, mouth-opening limitations, dysesthesia, and enlarged lymph nodes. All cases except one were surgically treated, and recurrence was found in two cases. Four of nine patients were followed up without recurrence and one patient underwent additional surgery due to recurrence. Conclusion: In our case series, notable symptoms included pain, edema, mouth-opening limitations, and dysesthesia; however, it was difficult to label these as specific symptoms of leiomyosarcoma. Considering the aggressive characteristics of the disease and poor prognosis, surgical treatment is necessary with careful consideration of postoperative radiotherapy and chemotherapy.